1. Field of the Invention
This invention relates to a whitening-resistant, highly melt-viscoelastic propylene copolymer and a process for producing the same. More particularly it relates to a propylene copolymer having a high stiffness, almost no impact- or folding-whitening, a superior impact resistance and also a high melt-viscoelasticity, obtained by using a specified Ziegler-Natta catalyst and also employing a four-stage ethylene-propylene copolymerization process, and a process for producing the same.
The present invention relates also to a propylene copolymer having a superior sheet post-processability (often referred to sheet moldability), blow moldability and injection moldability, and a process for producing the same.
2. Description of the Prior Art
Sheets produced by processing known polypropylene have drawbacks that sagging of the sheets at the time of heat molding for post-processing (or secondary processing) is rapid, the ranges of processing conditions are narrow, the molding efficiency is inferior, the above sagging is large in the case of sheets of broad width, the thickness of post-processed products is liable to be uneven, and weight wrinkles are liable to occur. Thus, only small type molded products can be produced.
On the other hand, when known polypropylene is used for blow molding, the following problems are raised:
(1) Since parison draw-down at the time of molding is large, the thickness of molded products is uneven. Thus, blow molding process can be applied only to small type commodities. PA1 (2) If a high molecular weight polypropylene is used in order to prevent the above drawn-down, there occur an inferior fluidity, a high load at the time of molding, a large energy loss and a danger of causing mechanical troubles, and further, molded products have an intense surface-roughening to lose their commodity value. In order to improve the above sheet-moldability and blow moldability of polypropylene, certain proposals have so far been made. For example, Japanese patent publication No. Sho 47-80614 (1972) and Japanese patent application laid-open No. Sho 50-8848 (1975) disclose a process of blending a low density polyethylene or the like to polypropylene. However, molded products using such a blend are liable to cause surface-roughening, and for preventing this, a powerful kneading is required at the time of melting; thus the process is restricted in the aspect of choice of kneader and power consumption. PA1 (1) the first stage wherein propylene or a mixed gas of ethylene with propylene, containing 0 to 7.5% by weight of ethylene, is fed to a reactor to form a polymer; successively, PA1 (2) the second stage wherein propylene or a mixed gas of ethylene with propylene, containing 0 to 15% by weight of ethylene, is fed to the reactor to form a polymer; PA1 (3) the amount of ethylene fed at the first stage and the second stage being in the range of 0.7 to 7.5% by weight based on the total amount of the mixed gas; PA1 (4) 60 to 89% by weight of the total polymerization amount (excluding soluble copolymer) being polymerized at the first stage and the second stage, and the proportion of the polymerization amount at the first stage to that at the second stage being in the range of 0.65:0.35 to 0.35:0.65; PA1 (5) between the intrinsic viscosities of the polymers formed at the first stage and the second stage, the intrinsic viscosity of the polymer having a higher molecular weight [.eta.].sub.H and that of the polymer having a lower molecular weight [.eta.].sub.L, having a relationship satisfying the following equation: EQU 3.0.ltoreq.[.eta.].sub.H -[.eta.].sub.L .ltoreq.6.5 (1); successively, PA1 (6) the third stage wherein ethylene or a mixed gas of ethylene with propylene, containing 70 to 100% by weight of ethylene, is fed to the reactor to polymerize 5 to 17% by weight of the total polymerization amount (excluding soluble copolymer); PA1 (7) the intrinsic viscosity of a polymer formed at the third stage [.eta.].sub.3 having a relationship satisfying the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.3 .ltoreq.[.eta.].sub.H ( 2); successively, PA1 (8) the fourth stage wherein a mixed gas of ethylene with propylene containing 40 to 70% by weight of ethylene is fed to the reactor to copolymerize 6 to 23% by weight of the total polymerization amount (excluding soluble copolymer); and PA1 (9) the intrinsic viscosity of the copolymer formed at the fourth stage [.eta.].sub.4 satisfying the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.4 .ltoreq.[.eta.].sub.H ( 3). PA1 (1) the first stage wherein propylene or a mixed gas of ethylene with propylene, containing 0 to 7.5% by weight of ethylene, is fed to a reactor to form a polymer, successively, PA1 (2) the second stage wherein propylene or a mixed gas of ethlene with propylene, containing 0 to 15% by weight of ethylene, is fed to the reactor to form a polymer; PA1 (3) the amount of ethylene fed at the first stage and the second stage being in the range of 0.7 to 7.5% by weight based on the total amount of the mixed gas; PA1 (4) 60 to 89% by weight of the total polymerization amount (excluding soluble copolymer) being polymerized at the first stage and the second stage, and the proportion of the polymerization amount at the first stage to that at the second stage being in the range of 0.65:0.35 to 0.35:0.65; PA1 (5) between the intrinsic viscosities of the polymers formed at the first stage and the second stage, the intrinsic viscosity of the polymer having a higher molecular weight [.eta.].sub.H and that of the polymer having a lower molecular weight [.eta.].sub.L, having a relationship satisfying the following equation: EQU 3.0.ltoreq.[.eta.].sub.H -[.eta.].sub.L .ltoreq.6.5 (1); successively, PA1 (6) the third stage wherein ethylene or a mixed gas of ethylene with propylene, containing 70 to 100% by weight of ethylene, is fed to the reactor to polymerize 5 to 17% by weight of the total polymerization amount (excluding soluble copolymer); PA1 (7) the intrinsic viscosity of a polymer formed at the third stage [.eta.].sub.3 having a relationship satisfying the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.3 .ltoreq.[.eta.].sub.H ( 2); successively, PA1 (8) the fourth stage wherein a mixed gas of ethylene with propylene containing 40 to 70% by weight of ethylene is fed to the reactor to copolymerize 6 to 23% by weight of the total polymerization amount (excluding soluble copolymer); and PA1 (9) the intrinsic viscosity of the copolymer formed at the fourth stage [.eta.].sub.4 satisfying the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.4 .ltoreq.[.eta.].sub.H ( 3). PA1 (1) at the first stage, a mixed gas of ethylene with propylene containing 1.5 to 4% by weight of ethylene is fed to the reactor to form a copolymer; successively, PA1 (2) at the second stage, a mixed gas of ethylene with propylene is fed to the reactor to form a copolymer; PA1 (3) the amount of ethylene fed at the first stage and the second stage is in the range of 1.5 to 4.0% by weight based on the total amount of the mixed gas; PA1 (4) 70 to 86% by weight of the total polymerization amount (excluding soluble copolymer) is polymerized at the first stage and the second stage, and the proportion of the polymerization amount at the first stage to that at the second stage is in the range of 0.65:0.35 to 0.35:0.65; PA1 (5) between the intrinsic viscosities of the polymers formed at the first stage and the second stage, the intrinsic viscosity of the polymer having a higher molecular weight [.eta.].sub.H and that of the polymer having a lower molecular weight [.eta.].sub.L have a relationship satisfying the following equation: EQU 3.0.ltoreq.[.eta.].sub.H -[.eta.].sub.L .ltoreq.6.5 (1); successively, PA1 (6) at the third stage, a mixed gas of ethylene with propylene containing 80 to 95% by weight is fed to the reactor to copolymerize 7 to 14% by weight of the total polymerization amount (excluding soluble copolymer); PA1 (7) the intrinsic viscosity of a polymer formed at the third stage [.eta.].sub.3 has a relationship satisfying the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.3 .ltoreq.[.eta.].sub.H ( 2); successively, PA1 (8) at the fourth stage, a mixed gas of ethylene with propylene containing 45 to 55% by weight of ethylene is fed to the reactor to copolymerize 8 to 15% by weight of the total polymerization amount (excluding soluble copolymer); and PA1 (9) the intrinsic viscosity of the copolymer formed at the fourth stage [.eta.].sub.4 satisfies the following equation: EQU [.eta.].sub.L .ltoreq.[.eta.].sub.4 .ltoreq.[.eta.].sub.H ( 3). PA1 Young's modulus: ASTM D882 (Kg f/mm.sup.2) PA1 Punching impact strength: ASTM D781 (Kg f/cm) PA1 Folding whitening: According to Chisso method (mm); when a sheet was folded and whitening began to occur, the folded radius at that time was measured. PA1 Impact whitening: According to Chisso method; i.e. a piece of an injection-molded product of 50 mm.times.50 mm.times.0.4 mm (thick) was contacted with a point of impact having a semisphere of 3.2 mm in radius at its tip end, using a Du Pont impact tester at 23.degree. C.; 200 g of a weight was dropped thereon from a height of 50 cm; and the diameter of whitened part (mm) was measured. PA1 Sheet appearance: Viewed by naked eyes. PA1 Heating behavior: According to Chisso method; in order to evaluate the heating vacuum formability of a sheet in a model manner, the sheet was fixed onto a frame of 40 cm.times.40 cm and placed in a constant temperature room at 200.degree. C. to observe the following physical properties: (i) sag amount of sheet (mm), (ii) maximum recovery amount (note: {1/150.times.(150-sag amount at the time of maximum recovery).times.100}), and (iii) retention time since the time of maximum recovery till the time of sag reopening. PA1 Bending modulus: JIS K 6758 (Kg f/cm.sup.2) PA1 Tensile strength: JIS K 6758 (Kg f/cm.sup.2) PA1 Hardness (Rockwell): JIS K 6758 (R-scale) PA1 Izod impact strength (II): JIS K 6758 (Kg f cm/cm) PA1 Spiral flow: Injected into a spiral form mold having a semi-circle of 6 mm in diameter and a pitch of 16 mm at a resin temperature of 250.degree. C., an injected resin pressure of 500 Kg/cm.sup.2 and a mold temperature of 50.degree. C., to measure the length of injected resin (cm). PA1 Whitening: This is almost the same as in the case of sheet except that the weight, height and the radius of the tip end of the point of impact were changed to 500 g, 1 m and 6.3 mm, respectively.
Further, Japanese patent application laid-open No. Sho 56-70014 (1981) proposes a two-stage copolymerization process wherein a difference in molecular weight and a difference in polymer amount are afforded between the respective polymer parts formed at the respective stages. However, copolymers obtained according to the process have insufficient flow characteristics at the time of melting.
Still further, Japanese patent application laid-open No. Sho 55-118906 (1980) discloses a process wherein the relationship between the melt swelling ratio (hereinafter abbreviated to SR) of polypropylene and its melt flow rate is made definite. In the case of this process, however, the relationship between the melt flow rate and the flow characteristics at the time of melting has not been taken into consideration, and SR, too, is not univocally determined in its relation to the extrusion shear rate of molten polypropylene; hence the process cannot always correspond to the improvements in various processing characteristics relative to the present invention.
Further, crystalline polypropylene (hereinafter abbreviated to polypropylene) obtained by polymerizing propylene by the use of a stereoregular catalyst has superior physical properties in stiffness, heat resistance, etc. On the other hand, there is a problem that its impact strength, particularly the impact strength at low temperatures is low, and in this respect, the range of its practical use has been restricted. In order to overcome this drawback, a number of processes wherein propylene and ethylene or another .alpha.-olefin are block-copolymerized have been proposed. According to these copolymerization processes, it is possible to obtain propylene-.alpha.-olefin copolymers having a superior low temperature impact resistance without damaging stiffness, heat resistance, etc. which are superior characteristics of polypropylene. On the other hand, as to such copolymers, a new drawback in physical properties as described below occurred, that is, a drawback that when molded products are produced using such copolymers, or molded products are transported or used, the impact or folding force loaded on the molded products readily whitens the loaded parts. The thus whitened molded products naturally lose their commodity value. As to processes for overcoming such a drawback of propylene-.alpha.-olefin block copolymers (hereinafter abbreviated to block copolymers), a number of proposals have been made. For example, (1) Japanese patent application laid-open Nos. Sho 55- 58245 (1980), Sho 55-10433 (1980), Sho 56-72042 (1981) and Sho 57-13741 (1982), etc. are directed to processes of blending polyethylene with block copolymers. These processes exhibit an effectiveness of overcoming the whitening, which, however, is still insufficient, and industrially there is raised a problem as to a uniformly blending process and further, blending cost, too, cannot be disregarded. Next, (2) Japanese patent publication Nos. Sho-47-26190 (1972) and Sho 49-24593 (1974) and Japanese patent application laid-open No. Sho 58-15548 (1983) disclose a multi-stage polymerization wherein propylene is homopolymerized at the first stage, and successively ethylene and propylene are copolymerized in a multi-stage manner. In these cases, however, as to the resulting block copolymers, reduction in stiffness can be prevented, but the effectiveness of overcoming the whitening is still insufficient. Next, (3) Japanese patent application laid-open No. Sho 54-40895 (1979) discloses a process for producing ethylene-propylene block copolymers wherein at the stage where ethylene-propylene copolymer part is prepared in the production of the copolymers, the hydrogen concentration is raised to thereby reduce the corresponding molecular weight to the part. However, the resulting block copolymers are insufficient in the improvement of low temperature impact strength. Next, (4) Japanese patent application laid-open Nos. Sho 54-13963 (1979), Sho 55-16048 (1980), Sho 56-55416 (1981), Sho 57-34112 (1982) and Sho 57-67611 (1982) disclose a process for a multi-stage copolymerization of ethylene with propylene wherein a small amount of ethylene is fed in the preparation process of propylene polymer at the first stage, and polymerization for ethylene-propylene copolymer is carried out successively at a multi-stage since the second stage. However, the thus obtained block copolymers are reduced in the stiffness and heat resistance which are characteristics intrinsic of polypropylene, to a large extent; hence the above various processes, too, cannot be said to be preferable.
On the other hand, a number of processes of improving the stiffness of block copolymers mainly by catalyst improvements have been proposed. Namely (5) Japanese patent publication Nos. Sho 47-8207 (1972), Sho 49-13231 (1974) and Sho 49-13512 (1974) have proposed an improved process of adding a third component to catalyst. Further, (6) Japanese patent application laid-open Nos. Sho 55-764 (1980), Sho 54-152095 (1979) and Sho 53-29390 (1978) and Japanese patent publication No. Sho 55-8011 (1980) have proposed an improved process using a specified catalyst. However, the proposals of the above (5) and (6) are directed to a technique for making the reduction extent of the stiffness of the resulting block copolymers as small as possible, as compared with polypropylene (homopolymer), so to speak, for relieving the drawback, but have not yet come to enable the polymers to acquire the same or higher level of stiffness as that of the homopolymer.